This invention relates generally to improvements in controlling growth processes of single crystal semiconductors for use in the manufacture of electronic components and, particularly, to a closed loop method and system for accurately controlling taper growth in a Czochralski crystal growth process.
Monocrystalline, or single crystal, silicon is the starting material in most processes for fabricating semiconductor electronic components. Crystal pulling machines employing the Czochralski process produce the majority of single crystal silicon. Briefly described, the Czochralski process involves melting a charge of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace. After the heated crucible melts the silicon charge, a crystal lifting mechanism lowers a seed crystal into contact with the molten silicon. The mechanism then withdraws the seed to pull a growing crystal from the silicon melt.
After formation of a crystal neck, the typical process enlarges the diameter of the growing crystal by decreasing the pulling rate and/or the melt temperature until a desired diameter is reached. By controlling the pull rate and the melt temperature while compensating for the decreasing melt level, the main body of the crystal is grown so that it has an approximately constant diameter (ie., it is generally cylindrical). Near the end of the growth process but before the crucible is emptied of molten silicon, the process gradually reduces the crystal diameter to form an end cone. Typically, the end cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt. During the growth process, the crucible rotates the melt in one direction and the crystal lifting mechanism rotates its pulling cable, or shaft, along with the seed and the crystal, in an opposite direction.
Although presently available Czochralski growth processes have been satisfactory for growing single crystal silicon useful in a wide variety of applications, further improvements are still desired. For example, a consistent and repeatable seed-end taper shape will help maintain a consistent value of maximum thermal stress, consistent heat transfer in the early body growth and improved reliability of diameter measurement systems. For these reasons, taper growth control for growing the taper to a repeatable shape is desired to improve taper consistency and repeatability.
Conventional taper growth control often involves controlling the heat into the melt by trial and error tuning. In the alternative, the heat is controlled by a control method, such as controlling a given measured temperature based on a preset temperature profile (i.e., target temperature vs. taper diameter). U.S. Pat. No. 5,223,078 and U.S. Pat. No. 4,973,377,the entire disclosures of which are incorporated herein by reference, describe conventional taper growth control.
For example, Maeda et al., U.S. Pat. No. 5,223,078 teaches a method of controlling the growth of the conical portion of the crystal adjacent the seed crystal (i.e., the taper). This method requires the active measurement and adjustment of process variables during growth of the taper. In the Maeda method, the melt temperature and diameter of the taper of the crystal being grown are measured. The change rate of the diameter is calculated and this change rate together with the measured temperature are compared to preset target temperature and change rate values. The target temperature is then determined again based on existing data from a target temperature data file and a target diameter change rate data file. The amount of electricity supplied to the heater is then controlled by a proportional-integral- derivative (PID) controller to obtain the corrected target temperature. In this manner, Maeda et al. attempt to make the length of the taper as short as possible.
Katsuoka et al., U.S. Pat. No. 4,973,377 describes a method of controlling the diameter of the taper by controlling the melt temperature and the rotational speed of the crucible. In this method, the control range of the crucible's rotational speed is made narrower as the diameter of the tapered portion approaches closer to that of the crystal's body portion and is made constant while the body portion is grown.
These approaches, however, are not entirely satisfactory. In U.S. Pat. No. 5,223,078, we are taught to use radiation thermometry as a secondary feedback to control melt temperature. This method has been tried, and failed, due to infrequent pyrometry blockage during machine set-up, SiO accumulation in the radiation viewpath and differences in pyrometer gain from device to device, for example. In addition to this same method of temperature control, U.S. Pat. No. 4,973,377 teaches a method of adjusting crucible rotation rate to adjust the melt temperature. The adjustment of the crucible rotation rate changes the radial flow velocity of the melt, and thereby introduces additional control dynamics, potentially destabilizing the control of the diameter. Additionally, as the modification of the crucible rotation changes the diffusion layer thickness of oxygen in the melt at the crucible wall, adjusting the crucible rotation rate will also adjust the oxygen content incorporated into the crystal. Since control of the oxygen is generally to a specification of a customer, it is not desired to modify the oxygen content in order to control the taper shape. Conversely, it is desired to control the taper shape independently of oxygen concentration adjustments. Moreover, the control techniques taught by these patents fail to provide adequate taper growth repeatability.
For these reasons, a method and system of controlling taper growth is desired that involves a preset temperature profile in combination with adjustment of pull speed to provide taper growth control for growing the taper to a repeatable shape to improve taper consistency and repeatability.